As a fixing device used for an electrophotographic image forming apparatus such as a copy machine, a printer, or a facsimile, conventionally, a roller fixing device has been widely used which is by a heat roller fixing method. In the method, a fixing rubber roller, the fixing rubber roller having an elastic layer and maintained at a predetermined temperature, and a press rubber roller, the press rubber roller having an elastic layer and in press contact with the fixing rubber roller, heat a transfer medium on which an unfixed toner image has been formed, while sandwiching and conveying the transfer medium.
However, with this type of roller fixing device, since the fixing rubber roller has a large thermal resistance and a large thermal capacity, warming up of the apparatus takes a long time, and the inner portion of the elastic layer rises in temperature, shortening the life of the fixing rubber roller.
Further, color images often have a large solid area, which is mostly in a color other than black, having a problem that a glaze mark, if there is, stands out.
For color image fixing requiring high glaze uniformity, a fixing member needs elasticity at a part in contact with an image so that a fixing pressure is evenly applied. Therefore, a fixing roller of a metal core roller formed with a rubber layer around the circumference has been used as the fixing member.
Specifically, a fixing device is disclosed which allows fixing of color toner without applying oil and seldom causes offset (see Patent Document 1).
There is also disclosed a fixing device which efficiently heats a heat fixing roller with an external heating unit from outside, and prevents, as much as possible, the fixing roller from dropping in temperature due to contact with a recording sheet or the like (see Patent Document 2).
There is further known a heat fixing device which prevents a sudden drop in the surface temperature of a heat fixing member just after rotation start, thus causes no fixing failure of a toner image even setting the surface temperature of the heat fixing member during waiting time to a low value, thereby always achieving a fixed image with high quality (Patent Document 3).
[Patent Document 1] TOKKAIHEI No. 10-254266
[Patent Document 2] TOKKAIHEI No. 11-52768
[Patent Document 3] TOKKAIHEI No. 11-38.802
However, in the above fixing devices, heat supply amount increases with speedup, the temperature difference between the surface of a rubber layer having a low thermal conductivity and the boundary surface of a metal core becomes further greater, and thus the durability of rubber at the boundary surface greatly degrades.
To avoid this problem, increasing the thermal conductivity of the rubber layer and using a large sized roller may be considered, but it is limited technically, in cost, or in practice.
Therefore, a method has been offered by which the surface of a fixing roller is heated, not from a metal core, but with an external heat roller or the like. However, in this method, it is difficult to make the contact area large, and also, the thermal capacity of the external heat roller increases, having a problem of incapability for shortening a preliminary heating time. Further, improved technologies as solution of these problems have been offered, but not satisfactory.
The invention solves the above described problems, and has an object to provide an image forming apparatus having a fixing device with high productivity, durability performance, and the stability of image quality.